The present invention relates to a cleaning method and a cleaning device for installation component parts that carry an electrical high-voltage.
Components in electric power supply facilities as e.g. components in transformer and switchboard stations are contaminated by dirt due to the influence of their operation, their environment and special events (such as e.g. fires). The dirt and the adherent contamination are of various nature. The possible range of dirt and contamination starts at slightly adherent, powdery, inorganic or organic dirt and extends over oils, fatty matters, liquid films and so-called biological films consisting of fungi and algae to nearly burnt-in residues consisting of metals, metal oxides and carbon which arise due to spark discharges and electric arcs.
Component parts of such facilities must be cleaned from time to time in order to maintain reliability of operation of the installations. For example, even if they have only a low conductivity, electrically conducting adherents at the surface of a ceramic isolator can decrease the isolating effect of the isolator. In extreme cases the can give rise to an electric arc and so at least for a short time cause an operation breakdown.
Consequences of such operation breakdowns range from short-time power interruptions to fires in an installation.
The well-known physical and chemical cleaning methods can be employed for cleaning. However, then, for the personal protection of the cleaning personnel, the operation of installations must usually be stopped and they must be powered down, i.e., it must be guaranteed that the electric high-voltage is disconnected. At least during the cleaning process this requires an operation shut-down which is economically disadvantageous and, moreover, often causes technical problems. The economic damage that electric supply and industry companies suffer due to the shut-down time required for cleaning high-voltage installations is important and would justify a considerable additional expenditure for the cleaning method in order to avoid shut-down.
Chemical cleaning methods are based on the effect that the dirt particles adhering to a component part are subjected to a chemical reaction by the cleaning agent thus being removed from the component part. Cleaning methods employing chemical cleaning agents usually leave behind liquid or solid residues that can be a risk to the operation safety of the installation depending on the nature of the residues. The residues themselves can play the role of a kind of contamination and can influence the isolation effect of component parts or they can develop corrosion at component parts. Thus, cleaning agents themselves must usually be expensively removed. This results in complicated and time-consuming cleaning processes.
Methods operating exclusively physically do not suffer from these disadvantages. These methods remove the contamination purely mechanically by abrasion from the component part. However, their cleaning often is less effective especially when oils and fatty matters are involved. In such a method, e.g. a high-pressure water jet directed to the component parts to be cleaned is employed for cleaning. Such wet-cleaning methods have severe drawbacks: on the one the high humidity can develop corrosion at the component parts and on the other hand dirty and thus contaminated waste water arises which must be disposed of or reprocessed. Without additional detergents or solvents it is only partly possible to remove fatty or oily residues. And finally, water has a relatively high electric conductivity. Thus, cleaning at operational voltage without exposing the cleaning personnel to danger is only possible in a low-voltage range (i.e. the range below 1 kV).
In a wider sense, particle blast methods as e.g. sand blasting can be classed among mechanical cleaning methods. Most of these methods (more precisely, most blast particle media) exhibit a strong abrasive effect that impairs the surface of the component parts to be cleaned.
A certain exception is the application of dry-ice particles as blast medium (i.e. particles of carbon dioxide in solid phase) as it is known from German patent applications DE 195 44 906 A1 and 196 24 652 A1, for example. Dry-ice particles are relatively soft (their hardness is similar to that of calcium sulphate) and so they do not damage the surface. Meanwhile, the application of solid carbon dioxide as blast medium for cleaning is quite common. Moreover, the cleaning is not only effected by the kinetic energy of the dry-ice particles impacting onto the surface, rather there are other contributing factors. So, the dry-ice particles sublime either upon or immediately after impact. The relatively high sublimation heat required is taken away from the impact point, thus locally strongly cooling the impact surface and the dirt adhering to it. The resulting thermal stress weakens the bondage between the dirt or contamination coating and the surface of the component parts to be cleaned. The contamination's freezing and embrittlement also reduces its adhesive strength. Finally, the sudden sublimation of dry-ice particles is a nearly explosive volume increase by a factor of about 600 blasting off the already loosened dirt.
A great advantage of such cleaning methods with solid carbon dioxide is the fact that dry-ice particles sublime to carbon dioxide in gaseous phase completely and without residual matter. Thus, no additional contaminated waste is produced. The waste to be disposed of consists only of the removed dirt and contamination.
Unfortunately, the instruments and methods for cleaning with dry-ice particles as, e.g., they are known from the previously mentioned documents cannot directly be employed for cleaning high-voltage installations that are not powered down since neither the personal protection of the personnel nor the safety of the instrument against high-voltage is provided. So, the cleaning workers must approach the installation to be cleaned to much, so that the danger of a high-voltage flashover arises.
Moreover, two important problems that can arise in principle must be taken into consideration, namely condensing atmospheric humidity that creates an additional conductivity and the effects of the removed dirt particles.
One must expect that by feeding the extremely cold dry-ice particles (the sublimation point of carbon dioxide lies at −78° C.) the humidity contained in the ambient air and eventually in the pressure gas will condense and thus reduce the isolating power of the ambient air. Especially for indoor installations, this could have fatal effects since their isolation distances have not been designed for condensing humidity. Thus, this could give rise to flashovers and electric arcs which would not only endanger the installation safety but also the cleaning personnel's safety. Since the minimum safety distances have been designed for normal installation operation but electric arcs could bridge a wider range, even when working at distance cleaning personnel would run a considerable risk of being hurt especially by scalds.
An other point of danger is the fact, that possibly the pressure air transporting the carbon-ice particles contains humidity that could induce a certain conductivity, thus, endangering the cleaning personnel as well as the cleaning device.
The second problem are the removed dirt particles. The dry ice will not simply be sprayed over the installation, rather it impacts at the surface to be cleaned with high kinetic energy and there looses the dirt particles. As already mentioned before, these often consist of combustible and partly of electrically conductive materials. When being finely divided in the ambient air of a high-voltage installation, it must be expected that they reduce the isolation power and could themselves cause electric arcs or could intensify the effects of electric arcs. Even dust explosions cannot not be excluded, rather they must be expected.
The importance of these dangers strongly depends on the actual installation especially on the height of the applied high-voltage. What would be no or nearly no problem in a 3 kV installation and only a slight problem in a 30 kV installation could grow to a deadly danger in a 300 kV installation.
Therefore, it is a objective of the present invention to provide a cleaning method and a cleaning device employed for that method that allow to clean high-voltage carrying installation component parts in a simple and for the operator as well as for the device safe way without the requirement of powering down said installation component parts.